Plane & Pilot
Friday, July 1, 2005

Saving Money On Fuel

With the price of avgas at record highs, here are some thoughts on getting the most out of your budget

I was told when I bought my first single-engine airplane back in the last century that I could estimate my total hourly operating cost by multiplying fuel expense by three. In those days, I flew a Globe Swift that burned six gallons an hour. Fuel was only about 70 cents per gallon as I remember, so I figured my fuel cost at $4.20 per hour and total cost to operate the Swift at a whopping $13 per hour, an intimidating number in those days." />

The obvious option of burning less fuel to fly the same distance doesn’t require high-order math. Every pilot knows most airplanes are more efficient when operated at lower power settings and lower speeds.

The myth is that aerodynamically clean machines, the airplanes you buy to fly fast in the first place, realize greater benefit from reduced power settings. We compared cruise speeds and fuel burns of a 1969 Mooney Executive, a 1979 Cessna 172N Skyhawk and a 1986 Beech F33A Bonanza, then calculated the relative percentage of performance and fuel burn. As it turns out, clean designs seem to benefit about the same as high-drag aircraft from lower power settings.

The results suggest that regardless of what you fly, you may realize a disproportionate increase in miles per gallon by flying slower with less power. (Duh!) The improvement in economy is obviously greatest at the lowest power setting. Perhaps surprisingly, the Skyhawk offers the best economy at 45%, providing 80% of its max cruise speed in exchange for only 63% of the fuel burn.

Flying higher also can benefit fuel economy. Many pilots get into the habit of operating at the same altitude all the time, irrespective of wind direction and velocity. Higher can be better for a number of reasons. First, the air is thinner up high, so an airplane turns in more speed for each gallon burned, no matter what the power setting. Most normally aspirated general aviation engines bleed down to 75%, the usual max cruise setting, at 7,000 to 9,000 feet. Power typically drops to 55% at 11,500 to 13,500 feet.

Second, flying higher usually means you’ll be up in smoother air, and there’s little question smoother is faster and more efficient. Finally, you may realize better winds up high. If they don’t happen to be going your direction, you always have the option to stay low.

Another common myth is that climbing to high altitude doesn’t pay unless you’re flying a long distance. Every airplane is different, so it’s impossible to generalize, but do the math for your model and you may find you’re money ahead climbing to 9,000 or 10,000 feet for even a 100 nm trip. If I don’t think about it, I tend to drive my Mooney around at 6,500 feet or less on trips less than 100 miles. It’s true my airplane hasn’t read the manual, so it doesn’t realize the book-promised fuel burns anyway, but as fuel prices have continued to climb, I’ve been climbing to at least 7,500 or 8,500 feet on short trips.


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